glass shows a pattern in high relief and gives a very brilliant effect.
The various varieties of rolled plate-glass are now produced for some purposes with a reinforcement of wire netting which is embedded in the mass of the glass. The wire gives the glass great advantages in the event of fracture from a blow or from fire, but owing to the difference in thermal expansion between wire and glass, there is a strong tendency for such “wired glass” to crack spontaneously.
Patent Plate-glass.—This term is applied to blown sheet-glass, whose surface has been rendered plane and brilliant by a process of grinding and polishing. The name “patent plate” arose from the fact that certain patented devices originated by James Chance of Birmingham first made it possible to polish comparatively thin glass in this way.
(B) Pressed Glass.—The technical difference between pressed
and moulded glass is that moulded glass-ware has taken its form
from a mould under the pressure of a workman’s breath, or of compressed
air, whereas pressed glass-ware has taken its form from a
mould under the pressure of a plunger. Moulded glass receives
Fig. 20.—Modern American Glass-Press.
the form of the
mould on its
interior as well as on
its exterior surface.
In pressed glass the
exterior surface is
modelled by the
mould, whilst the
interior surface is
modelled by the
plunger (fig. 20).
The process of pressing glass was introduced to meet the demand for cheap table-ware. Pressed glass, which is necessarily thick and serviceable, has well met this legitimate demand, but it also caters for the less legitimate taste for cheap imitations of hand-cut glass. An American writer has expressed his satisfaction that the day-labourer can now have on his table at a nominal price glass dishes of elaborate design, which only an expert can distinguish from hand-cut crystal. The deceptive effect is in some cases heightened by cutting over and polishing by hand the pressed surface.
The glass for pressed ware must be colourless, and, when molten, must be sufficiently fluid to adapt itself readily to the intricacies of the moulds, which are often exceedingly complex. The materials employed are sand, sulphate of soda, nitrate of soda, calcspar and in some works carbonate of barium. The following is an analysis of a specimen of English pressed glass; SiO2, 70.68%; Na2O, 18.38%; CaO, 5.45%; BaO, 4.17%; Al2O3, 0.33%; and Fe2O3, 0.20%. Tanks and pots are both used for melting the glass. The moulds are made of cast iron. They are usually in two main pieces, a base and an upper part or collar of hinged sections. The plunger is generally worked by a hand lever. The operator knows by touch when the plunger has pressed the glass far enough to exactly fill the mould. Although the moulds are heated, the surface of the glass is always slightly ruffled by contact with the mould. For this reason every piece of pressed glass-ware, as soon as it is liberated from the mould, is exposed to a sharp heat in a small subsidiary furnace in order that the ruffled surface may be removed by melting. These small furnaces are usually heated by an oil spray under the pressure of steam or compressed air.
See Antonio Neri, Ars vitraria, cum Merritti observationibus (Amsterdam, 1668) (Neri’s work was translated into English by C. Merritt in 1662, and the translation, The Art of making Glass, was privately reprinted by Sir T. Phillipps, Bart., in 1826); Johann Kunkel, Vollständige Glasmacher-Kunst (Nuremberg, 1785); Apsley Pellatt, Curiosities of Glass-making (London, 1849); A. Sauzay, Marvels of Glass-making (from the French) (London, 1869); G. Bontemps, Guide du verrier (Paris, 1868); E. Peligot, Le Verre, son histoire, sa fabrication (Paris, 1878); W. Stein, “Die Glasfabrikation,” in Bolley’s Technologie, vol. iii. (Brunswick, 1862); H. E. Benrath, Die Glasfabrikation (Brunswick, 1875); J. Falck and L. Lobmeyr, Die Glasindustrie (Vienna, 1875); D. H. Hovestadt, Jenaer Glas (Jena, 1900; Eng. trans. by J. D. and A. Everett, Macmillan, 1907); J. Henrivaux, Le Verre et le cristal (Paris, 1887), and La Verrerie au XX e siècle (1903); Chance, Harris and Powell, Principles of Glass-making (London, 1883); Moritz V. Rohr, Theorie und Geschichte der photographischen Objektive (Berlin, 1899); C. E. Guillaume, Traité pratique de la thermométrie de précision (Paris, 1889); Louis Coffignal, Verres et émaux (Paris, 1900); R. Gerner, Die Glasfabrikation (Vienna, 1897); C. Wetzel, Herstellung grosser Glaskörper (Vienna, 1900); C. Wetzel, Bearbeitung von Glaskörpern (Vienna, 1901); E. Tscheuschner, Handbuch der Glasfabrikation (Weimar, 1885); R. Dralle, Anlage und Betrieb der Glasfabriken (Leipzig, 1886); G. Tammann, Kristallisieren und Schmelzen (Leipzig, 1903); W. Rosenhain, “Some Properties of Glass,” Trans. Optical Society (London, 1903), “Possible Directions of Progress in Optical Glass,” Proc. Optical Convention (London, 1905) and Glass Manufacture (London, 1908); Introduction to section 1, Catalogue of the Optical Convention (London, 1905). (H. J. P.; W. Rn.)
History of Glass Manufacture.
The great similarity in form, technique and decoration of the earliest known specimens of glass-ware suggests that the craft of glass-making originated from a single centre. It has been generally assumed that Egypt was the birthplace of the glass industry. It is true that many conditions existed in Egypt favourable to the development of the craft. The Nile supplied a waterway for the conveyance of fuel and for the distribution of the finished wares. Materials were available providing the essential ingredients of glass. The Egyptian potteries afforded experience in dealing with vitreous glazes and vitreous colours, and from Egyptian alabaster-quarries veined vessels were wrought, which may well have suggested the decorative arrangement of zigzag lines (see Plate I. figs. 1, 2, 4 d) so frequently found on early specimens of glass-ware. In Egypt, however, no traces have at present been found of the industry in a rudimentary condition, and the vases which have been classified as “primitive” bear witness to an elaboration of technique far in advance of the experimental period. The earliest specimens of glass-ware which can be definitely claimed as Egyptian productions, and the glass manufactory discovered by Dr Flinders Petrie at Tell el Amarna, belong to the period of the XVIIIth dynasty. The comparative lateness of this period makes it difficult to account for the wall painting at Beni Hasan, which accurately represents the process of glass-blowing, and which is attributed to the period of the XIth dynasty. Dr Petrie surmounts the difficulty by saying that the process depicted is not glass-blowing, but some metallurgical process in which reeds were used tipped with lumps of clay. It is possible that the picture does not represent Egyptian glass-blowers, but is a traveller’s record of the process of glass-blowing seen in some foreign or subject country. The scarcity of specimens of early glass-ware actually found in Egypt, and the advanced technique of those which have been found, lead to the supposition that glass-making was exotic and not a native industry. The tradition, recorded by Pliny (Nat. Hist. xxxvi. 65), assigns the discovery of glass to Syria, and the geographical position of that country, its forests as a source of fuel, and its deposits of sand add probability to the tradition. The story that Phoenician merchants found a glass-like substance under their cooking pots, which had been supported on blocks of natron, need not be discarded as pure fiction. The fire may well have caused the natron, an impure form of carbonate of soda, to combine with the surrounding sand to form silicate of soda, which, although not a permanent glass, is sufficiently glass-like to suggest the
